Not long after large-scale, effective means for generating microwave energy were discovered (e.g. the discovery of the magnetron in the 1930's), it became apparent that such ample sources of microwave energy in the very high and ultrahigh frequency ranges could be employed in food technology, e.g. for rapid cooking of various foods. Frequencies in the megahertz range (e.g. above 800 megahertz) were found to be extremely effective in raising the temperature of foods, particularly when the food material contained substances such as water which are good absorbers of microwave energy. So-called microwave ovens have been in use for various cooking and thawing operation for many years.
One of the economically important applications of microwave heating technology involves the so-called "reconstitution" of frozen convenience foods, e.g. snack food items. Theoretically, conventional heating ovens can be dispensed with entirely in small restaurant operations, wherein food is stored in a frozen or refrigerated state for subsequent heating and consumption. Not only can the frozen or refrigerated food items by thawed very quickly, they can also, either in a plurality of operations or in the same operation, be brought to moderately elevated temperatures on a short-order basis.
Such short-order thawing and heating techniques are not equally effective for all foods, however. Some foods, particularly sandwiches are subject to undesirable changes in moisture content during the microwave "reconstitution" process. For example, excessive loss of moisture during thawing and/or heating of bread components can render them rubbery or leathery and hence less palatable. Both the texture and the taste of bread components and sandwich filling materials can be affected adversely by microwave "reconstitution".
Somewhat similar problems may be encountered in any fast cooking or heating or thawing process, e.g. with infrared heating and conventional hot air ovens. At the present time, the problems appear to be particularly acute when microwave energy is used for thawing or heating. There are great advantages in being able to heat frozen sandwich-type items directly from a cold state to provide hot snack items such as hamburgers, grilled cheese sandwiches, hot beef sandwiches, and the like. If the price to be paid for such advantages is a sandwich-like item which is distasteful, leathery, unduly moist, or otherwise unlike hot sandwiches prepared in a more conventional manner, the likelihood of realizing these advantages on a large commercial scale remains relatively remote.
It has been suggested that the effects of microwave heating can be controlled by special packaging of sandwich items. For example, U.S. Pat. No. 4,015,085 (Woods), issued Mar. 29, 1977 discloses a sandwich having a moisture containing bread base and a filler on top of the base enclosed in a non-metallic film, provided with means within this enclosure for controlling the effects of heating directly from the frozen state. The particular means for controlling these effects is a conductive layer between the bottom of the bread base and the non-metallic film. A moisture absorbent layer may also be included in the enclosure. The apparent purpose of the conductive layer is to redistribute the heat energy within the enclosure, partially shielding the bread base from heat while permitting full exposure of the sandwich filling material to heat. Microwave heating of the thus-packaged sandwich item is specifically contemplated.
Both frozen and non-frozen sandwich items and other filled foods items have received a considerable amount of attention in the U.S. patent literature. The following references are believed to be representative of the prior art in this specific field.
______________________________________ U.S. Pat. No. Patentee Issue Date ______________________________________ 3,189,463 Jones June 15, 1965 3,475,182 Goodman et al October 28, 1969 3,532,510 Zimmerman October 6, 1970 3,539,354 Colvin November 10, 1970 3,719,138 Blaetz et al March 6, 1973 ______________________________________
Of the foregoing references, the Zimmerman patent appears to be of interest for its disclosure of a filling which can be completely enclosed on all sides by a dough material.